1. Field of the Invention
The present invention relates to current switches such as may be part of an analog-to-digital or digital-to-analog converter circuit using delta modulation.
2. Description of the Prior Art
Current switches are often used in analog-to-digital and digital-to-analog converters which use delta modulation. In such converters, the logic level of the data and an associated clock signal determine the state of such a current switch, whose output is smoothed by means of an integrator and a low-pass filter to form the analog output signal. A current switch of this kind is described in "1974 IEEE International Solid-State Circuits Conference Digest of Technical Papers," February, 1974, pp. 192/193.
Such current switches are also suitable for demodulating pulse-duration-modulated signals; in which case, the low-pass-filtered output of the current switch is the demodulated output signal.
Such demodulators suffer from the drawback that the leading and trailing edges of the output current of the current switch affect the analog output signal, which is formed by integrating the switched current over several clock periods. Thus, edge distortions which do not average out in the integral will result in a spurious output, such as distortion or noise. This disadvantageous behavior is described, for example, in "Funk-Technik," 1975, pages 180 to 184, especially Chapter 1.5.3, page 184, with FIG. 9.
If, during the demodulation of a pulse-density-modulated signal, the required signal-to-noise ratio of, e.g., 85 dB for an audio signal of 0-16 kHz is to be maintained, it is required that, at a clock rate of 5 MHz, the effective time deviation per leading or trailing edge of the switched current, corresponding to the integral error, must not be greater than 100 ps. As a comparison, the corresponding aperture time of a weighted digital-to-analog converter with a 14-bit dynamic range at a 85-dB signal-to-noise ratio is about 500 ps.